1. Field of the Invention
The present invention relates to a liquid crystal-element such as a liquid crystal display element or an liquid crystal optical shutter and, more particularly, to a ferroelectric liquid crystal element and a liquid crystal apparatus using the same. More specifically, the present invention relates to a liquid crystal element and a liquid crystal apparatus using the same, in which display characteristics are improved by improving an orientation state of liquid crystal molecules.
2. Related Background Art
A display element for controlling a transmission beam by a combination with a polarization element by Utilizing refractive index anisotropy of ferroelectric liquid crystal molecules is proposed by Clark and Lagerwall (Japanese Laid-Open Patent Application No. 56-107216 and corresponding U.S. Pat. No. 4,367,924). This ferroelectric liquid crystal has a chiral smectic C phase (SmC*) or H phase (SmH*) having a non-helical structure. In this state, the ferroelectric liquid crystal has the first or second optically stable state in response to an electric field applied thereto. When no electric field is applied to this ferroelectric liquid crystal, it maintains the current state. In this manner, the ferroelectric liquid crystal has a bistable property and has a high response speed to a change in electric field. It is expected as a high-speed storage type display element in a variety of applications. In particular, this ferroelectric liquid crystal is expected for an application as a large, high-precision display due to the functions of the liquid crystal.
In order to enhance a predetermined drive characteristic of an optical modulation element using this bistable liquid crystal, the liquid crystal sealed between a pair of parallel substrates is required to be kept in a molecular orientation state to cause effective phase transition between the above two stable states.
A transmittance of a liquid crystal element utilizing a birefringence of the liquid crystal under the presence of crossed Nicols is given as follows:       I    /          I      0        =            sin      2        ⁢          xe2x80x83        ⁢    4    ⁢          xe2x80x83        ⁢    θ    ⁢          xe2x80x83        ⁢          sin      2        ⁢          xe2x80x83        ⁢                  δ        ⁢                  xe2x80x83                ⁢        nd            λ        ⁢          xe2x80x83        ⁢    π  
where I0 is the incident light intensity, I is the transmitted light intensity, xcex8 is the tilt angle, xcex94n is the refractive index anisotropy, d is the thickness of a liquid crystal layer, and xcex is the wavelength of incident light.
The tilt angle xcex8 in the non-helical structure appears as an angle of an average molecular axis of liquid crystal molecules having helical arrangements in the first and second orientation states. According to the above equation, a maximum transmittance can be obtained when the tilt angle xcex8 is given as 22.5xc2x0. The tilt angle xcex8 in the non-helical structure for realizing the bistable property must be as close as to 22.5xc2x0.
Of orientation methods of ferroelectric liquid crystals, a method capable of aligning a molecular layer consisting of a plurality of molecules constituting a smectic liquid crystal along a uniaxial direction along a normal to the molecular layer, and capable of achieving such orientation by simple rubbing is preferable.
An example of the method of aligning a ferroelectric liquid crystal and, particularly, a chiral smectic liquid crystal having a non-helical structure is described in U.S. Pat. No. 4,561,726.
The following problem is posed when the conventional orientation method and, particularly, an orientation method using a rubbed polyimide film is applied to a ferroelectric liquid crystal having a non-helical structure exhibiting the bistable property and announced by Clark and Lagerwall.
According to experiments of the present inventors, it is found that an apparent tilt angle xcex8 (i.e., xc2xd an angle formed by two molecular axes of the two stable states) of a ferroelectric liquid crystal having a non-helical structure aligned by a conventional rubbed polyimide film is smaller than a tilt angle (i.e., an angle xcex8 which is xc2xd a vertex angle of a triangular cone, shown in FIG. 4A to be described later) of a ferroelectric liquid crystal having a helical structure. In particular, the tilt angle xcex8 of the ferroelectric liquid crystal having the non-helical structure oriented by the conventional rubbed polyimide film generally falls within the range of about 3xc2x0 to 8xc2x0, and its transmittance falls within a maximum range of about 3% to 5%.
As can be apparent from the above description, according to Clark and Lagerwall, although the tilt angle of a ferroelectric liquid crystal having a non-helical structure for realizing the bistable property must be equal to the tilt angle of a ferroelectric liquid crystal having a helical structure, the tilt angle xcex8 of the non-helical structure is smaller than the tilt angle xcex8 of the helical structure. In addition, it is also found that a cause for setting the tilt angle xcex8 of the non-helical structure to be smaller than that (xcex8) of the helical structure is based on helical orientation of liquid crystal molecules in the non-helical structure. More specifically, in a ferroelectric liquid crystal having a non-helical structure, liquid crystal molecules are continuously twisted from an axis of the liquid crystal molecules adjacent to the upper substrate to the axis of the liquid crystal molecules adjacent to the lower substrate with respect to the normal to the substrates. For this reason, the tilt angle xcex8 of the non-helical structure is smaller than the tilt angle xcex8 of the helical structure.
Since a polyimide orientation film serving as an insulating layer is present between each electrode and the liquid crystal layer, when a voltage having one polarity is applied to the chiral smectic liquid crystal which is oriented by the conventional rubbed polyimide orientation film so as to switch the orientation state from the first optically stable state (e.g., a white display state) to the second optically stable state (e.g., a black display state), a reverse electric field Vrev having the other polarity is applied to the ferroelectric liquid crystal after the voltage having one polarity is withdrawn. The electric field Vrev causes an after image at the time of display.
This reverse electric field generation phenomenon is described in Akio Yoshida, xe2x80x9cSSFLC Switching Characteristicsxe2x80x9d, Lecture Papers on Liquid Crystal Forum, October, 1987, pp. 142-143.
One of the present inventors found the following phenomenon associated with an orientation state of a ferroelectric liquid crystal.
An LP 64 (tradename) having a relatively small pretilt angle available from TORAY INDUSTRIES, INC. or the like is applied to substrates to form orientation films thereon, and the substrates respectively having the orientation films are rubbed and adhered to each other with a gap of about 1.5 xcexcm such that the rubbing direction of one substrate is the same as that of the other substrate. A ferroelectric liquid crystal CS1014 (tradename) available from Chisso Kabushiki Kaisha is injected into the space defined between the two substrates of the obtained cell and is sealed. When the temperature of the ferroelectric liquid crystal is decreased, phase transitions shown in FIGS. 2A to 2E are obtained. More specifically, in a state shown in FIG. 2A immediately after the phase transition from a high-temperature phase to an Sc* phase, the ferroelectric liquid crystal takes orientation states (C1 orientation states) 21 and 22 having a small contrast value. When the temperature is further decreased and reaches a given temperature region, zig-zag defects 23 are generated, and orientation states (C2 orientation states) 24 and 25 having large contrast values with respect to these defects appear, as shown in FIG. 2B. When the temperature is further decreased, the C2 orientation state propagates (FIGS. 2C and 2D), and the entire liquid crystal is set in the C2 orientation state (FIG. 2E).
These C1 and C2 orientation states can be described in accordance with a difference in chevron structure of the smectic layer, as shown in FIG. 3. Smectic layers 31 in FIG. 3 are classified into C1 orientation regions 32 and a C2 orientation region 33.
A smectic liquid crystal generally has a layer structure. When the SA phase transits into an SC or SC* phase, the layer interval is reduced, and the layers have a structure bent at the center between the upper and lower substrates (i.e., the chevron structure), as shown in FIG. 3. The bending direction is determined by the C1 and C2 orientation states. However, as is known well, the bending direction is determined such that liquid crystal molecules at the substrate boundary form an angle with respect to the substrate surface (pretilt) and are inclined so that the heads of the liquid crystal molecules are inclined toward the rubbing direction (e.g., the leading ends float). Since the elastic energy of the C1 orientation state is not equal to that of the C2 orientation state, phase transitions occur at the predetermined temperatures described above. In addition, a phase transition may occur due to a mechanical stress.
When the layer structure of FIG. 3 is observed from the top, a boundary 34 from the C1 orientation state to the C2 orientation state in the rubbing direction has a zig-zag pattern and is called a lightening defect, while a boundary 35 from the C2 orientation state to the C1 orientation state in the rubbing direction forms a wide slow curve and is called a hairpin defect. There is provided a conventional liquid crystal element using the C2 orientation state of the C1 and C2 orientation states in favor of high contrast.
A liquid crystal apparatus using the ferroelectric liquid crystal element described above is taken into consideration. A matrix display apparatus having stripe electrodes on the inner surfaces of a pair of substrates used in the ferroelectric liquid crystal element so that the stripe electrodes are perpendicular to each other can be driven by methods disclosed in Japanese Laid-Open Patent Application Nos. 59-193426, 59-193427, 60-156046, and 60-156047.
FIG. 18 shows a drive waveform wherein periods of (1) a xe2x80x9cblackxe2x80x9d erase phase and (2) a selective xe2x80x9cwhitexe2x80x9d write phase are assigned to a selection period of one scanning line. In the period of the (1) erase phase, a positive voltage is applied to scanning electrodes to set all or predetermined pixels on the scanning line in the first stable state (to be referred to as a xe2x80x9cblackxe2x80x9d erase state hereinafter). In the period of the (2) selective xe2x80x9cwhitexe2x80x9d write phase, a negative voltage is applied to the scanning electrodes, and a positive voltage is selectively applied to only data electrodes corresponding to pixels (selection pixels) supposed to be inverted to the second stable state (to be referred to as a xe2x80x9cwhitexe2x80x9d state hereinafter). The negative voltage is applied to the data electrodes corresponding to the remaining pixels (semi-selection pixels). An inversion electric field having a threshold value or more is generated by the selection pixel, while an inversion electric field having a value smaller than the threshold value is generated by the semi-selection pixel. xe2x80x9cWhitexe2x80x9d is written in the selection pixel, while, a xe2x80x9cblackxe2x80x9d state is maintained in the semi-selection pixel. At this time, the absolute value of the erase phase voltage is always equal to that of the write phase voltage.
In a method of rubbing a polymer film of, e.g., polyimide or polyvinyl alcohol having a relatively small pretilt angle, as the most popular method of controlling orientation of a ferroelectric liquid crystal, the two orientation states (bistable, i.e., splay orientation) wherein the apparent tilt angle xcex8 is as small as about xc2xd the true tilt angle xcex8 are very important, as previously described.
When multiplex driving is applied to an element having these orientation states by using the drive waveform shown in FIG. 18, sufficiently high contrast cannot be obtained while a sufficient drive margin can be obtained. Therefore, much room for improvement in elements is left.
The present invention has been made in consideration of the conventional technical problems, and has as its object to provide a liquid crystal element having a higher contrast than that of the conventional element.
It is another object of the present invention to provide a ferroelectric liquid crystal element capable of obtaining a display for forming a large tilt angle xcex8 in a non-helical structure of a chiral smectic liquid crystal, displaying an image having a high contrast, and inhibiting formation of an after image.
It is still another object of the present invention to provide a ferroelectric liquid crystal element capable of assuring a sufficient drive margin.
It is still another object of the present invention to provide a liquid crystal apparatus having a means capable of performing a display having a sufficiently drive margin, a high contrast, and a high transmittance.